The invention relates to semiconductor devices. More specifically, the invention relates to FETS having lightly doped drains.
It is a never-ending goal of the semiconductor industry to increase the operating speeds of transistors. The speed of a Field Effect Transistor (FET) can be increased by shortening its channel length. As the channel length is shortened, the distance that carriers must travel between the FET's source and drain regions is reduced.
Device lifetime and reliability become issues of increasing concern as channel lengths of the FETs are shortened. A shorter channel length makes the FET susceptible to the effects of "hot carriers." Hot carriers are typically electrons that are accelerated by high electric fields within the channel. As the gradient of the electric field is increased, the probability of the hot carriers being injected into the thin dielectric layer of the FET's gate structure is also increased. Over time, these hot carriers create a permanent charge in the gate's dielectric. As a result, the gate operating characteristics are progressively degraded over the lifetime of the FET. Eventually, the FET is destroyed.
The effects of the hot carriers can be mitigated by forming a lightly doped drain (LDD) region contiguous with the more heavily drain region and optionally forming an LDD region contiguous with the more heavily doped source region. The LDD regions are formed below the gate oxide/substrate interface and closest to the point within the channel where the greatest electric field strength occurs. The LDD regions provide a series resistance between the drain and source regions which reduces the electrical field strength adjacent the oxide/substrate interface. This reduction in field strength directly reduces the transfer of energy to carriers at the interface. Although the speed of the carriers is slowed somewhat, the number of hot carriers injected into the gate dielectric is reduced dramatically and the lifetime of the FET gate is increased significantly.
The slope of the junction between the LDD region and the channel should be gradual in order to reduce the electric field strength and, consequently, the production of hot carriers. This requires the shape of the LDD region to be formed precisely.
However, problems exist with forming the junction. The LDD dopant, typically arsenic, does not diffuse rapidly when the FET n-channel is being annealed (even though several high temperature anneals are performed to activate the dopant). There is great difficulty in using diffusion of the LDD dopant to achieve a desired distribution. Thus, an abrupt or otherwise undesirably shaped junction results. Even with the addition of LDD regions, the imprecise formation of the junctions causes the lifetime of the FET to be reduced by the hot carriers.